If your visit to the Classic Rotary Phones Forum was enjoyable or helpful, you may
tell us by making a small donation. The donation can be made safely via PayPal
and will help to off-set the costs of the Forum operation. Thank you!

The phone is a remarkably complex, simple device,
and very rarely ever needs repairs, once you fix them. Dan/Panther

I've seen those a few times on off-brand 2500s, but never on a 500, and never on a Bell System phone. Besides business use as Ben mentioned, they're used on a lot of the Design Line phones like Exeter and Telstar.